EP0030890B1 - Process and apparatus for fluxless brazing - Google Patents

Description

The present invention relates to an improved method for brazing metal parts, as well as a device and a filler metal for the implementation of this method.

It relates more particularly to the brazing of parts, in particular aluminum tubes, and is of particular interest for the construction of heat exchangers.

The brazing of aluminum, without flux, is currently carried out most often by immersion of the part to be brazed in a bath of liquid filler metal and by the vibration of the bath: this vibration is generally carried out with using sonotrodes connected to an ultrasonic generator and fixed on the outside of the tank containing the filler metal. One such technique is described in particular in US Patent No. 3,680,200.

These conventional soldering methods have a number of drawbacks well known to technicians. These drawbacks, mainly due to a lack of homogeneity of the ultrasonic field, generally result in a lack of wettability causing local defects in the filler metal inside the solder, which weakens it until the make it defective.

Furthermore, especially in the case of aluminum brazing, generally used as filler metal, alloys of zinc and aluminum and beryllium, as in French patent No. 2195 501, or even alloys of aluminum and yttrium, germanium, indium, silver, as in French patent No. 2 201 950, or other conventional alloys used for welding or brazing aluminum.

In general, the aim is to avoid any contamination of the brazed area, any overheating harmful to the parts to be brazed and to the equipment used.

The important character is the wetting of the metal of the part to be brazed by the molten filler metal.

It appeared that it is more important according to the invention that the ultrasound is applied to the parts to be brazed rather than to the brazing bath, because thus the ultrasonic energy is concentrated where it is most useful, that is to say brdire instead of solder.

The object of the invention is to overcome the defects and drawbacks of conventional methods, devices and filler metals.

It relates to a process for the brazing without flux of metal parts in which the parts immersed in a molten bath of filler metal are subjected to the action of ultrasound before removing them from said bath and allowing them to cool, characterized in that said parts are first placed in intimate contact with a sonotrode and then the parts and sonotrode are immersed together in the molten bath before emitting ultrasound by said sonotrode.

For the implementation of such a method applied to the brazing of aluminum parts, it is particularly advantageous to use a filler metal consisting of zinc, or cadmium, or an alloy containing at least zinc and cadmium, and more particularly of a binary alloy containing approximately 66% of cadmium and 34% of zinc, melting approximately between 265 and 305 ° C.

The part of the sonotrode called to be in contact with the filler metal bath is advantageously made of unalloyed titanium, which is passive by thermal oxidation, before the first use.

The invention also relates to a device for brazing implementing such a method characterized in that it comprises a frame supporting a crucible for a bath of molten filler metal, the bottom of the crucible being leaktightly traversed by a sonotrode sliding under the action of a jack fixed on the frame, with an ultrasound generator associated with the sonotrode, and heating means, controlled, associated with this crucible, the active end of the sonotrode being dug at least one imprint of complementary shape of at least a part of the external surface of the parts to be brazed, and constituting a recess of dimensions slightly larger than those of the external envelope of the metal parts putting said parts in intimate contact with the sonotrode, the cylinder alternately moving the active end in and out of the bath. Document US-A-3,752,381 discloses such a device for brazing comprising a frame supporting a crucible for a bath of molten filler metal, the bottom of the crucible being traversed in a sealed manner by a sonotrode movable in and out bath under the action of a jack fixed on the frame, with an ultrasonic generator associated with the sonotrode and controlled heating means associated with the crucible, the active end of said sonotrode being dug with at least one imprint.

In such a device for the brazing of U-shaped tube stocks, the cavity hollowed out in the active end of the sonotrode advantageously has the shape of a semi-toric recess with U-shaped generator of dimensions slightly greater than those of the outer shell of the sticks.

This active end is also preferably made of passivated unalloyed titanium. An amplifier, consisting of a profiled metal spacer, advantageously made of titanium, is disposed between the ultrasound generator and the sonotrode.

In order to protect the ultrasonic generator from excessive heating, it is advantageous to have a cooler, in particular with blown air, between the sonotrode and the ultrasonic generator.

In such a device, it is advantageous according to the invention that the passage of the sonotrode through the bottom of the crucible is in the position of work of the sonotrode at an ultrasonic vibration node, while the joints between sonotrode, amplifier and ultrasonic generator are conventionally located at ultrasonic vibration bellies.

It is preferable, according to the invention, that seals with a high ultrasonic absorption coefficient line the passage of the sonotrode through the bottom of the crucible, in order to avoid any parasitic propagation of ultrasound in the crucible.

The invention also relates to a filler metal for the brazing of aluminum parts according to the method and / or the device of the invention, characterized in that it consists of an alloy containing at least zinc and cadmium, with a melting point of less than about 450 ° C.

• la figure 1 est une vue en élévation, de profil, d'un dispositif de brasage d'une crosse de deux tubes en U suivant l'invention, et,
• la figure 2 est une vue de face, analogue à celle de la figure 1, du même dispositif, avec certaines parties en coupe.

The invention will be better understood on reading the detailed description which follows and on examining the appended drawings which represent by way of nonlimiting example an embodiment of the invention. In these drawings:

• FIG. 1 is a view in elevation, in profile, of a device for brazing a stock of two U-shaped tubes according to the invention, and,
• Figure 2 is a front view, similar to that of Figure 1, of the same device, with certain parts in section.

The device shown in the drawings essentially comprises a rigid frame 1, vertical, terminated at the top by a bracket 24 which supports a crucible 2 containing the molten filler metal up to a level indicated by a broken line at 25. This crucible 2 is heated by conventional annular external electrical resistors8. The temperature inside the bath is controlled by a thermal probe 9.

Inside the crucible, a brazing assembly 26 consisting of two tubes, the ends of which are force-fitted respectively into the arms of a stick, is clamped by a flange 15 which centers it, and which slides freely inside the crucible 2.

The U-shaped end of the butt 26 is supported in a recess 17 of complementary shape hollowed out in the active end 16 of a sonotrode 6 made of titanium.

This sonotrode 6 slides in leaktight fashion through a cable gland fitted with sealing rings 14 made of a material resistant to temperature and a high ultrasonic absorption coefficient such as graphite asbestos, which prevents any parasitic propagation of ultrasound. in the crucible.

This sonotrode 6 is made integral with an ultrasonic amplifier 7, also made of titanium, by an axial screw 22. The amp) ifier7 is also made integral by an axial screw 21 with an ultrasonic generator 10 advantageously constituted by a piezoelectric converter connected to a suitable electricity source by a pipe 18. Before the first use, the sonotrode 6 is subjected to passivation by oxidation by heating in ambient air and at atmospheric pressure at 450 ° C. approximately for 24 hours. This treatment makes the sonotrode practically insensitive to corrosion under conditions of use.

A cooler 4, constituted by a metal sheath surrounding the lower part of the sonotrode 6 and the upper part of the amplifier7, allows cooling under the action of an air jet supplied by a tube 19. The amplifier is made integral, by radial screws 23, with a plate 27 connected by four threaded rods 11 to a flange 3 fixed to the end of a spacer 12, integral with the piston of a jack 13 fixed by lugs 20 to the frame 1.

In order to ensure good transmission of ultrasound from the generator 10 to the amplifier 7, and to the sonotrode 6, the total length of the amplifier 7, as well as the length of the sonotrode 6, are multiples of the half-length. wave wave emitted by the generator 10.

Similarly, the screws 23 are located at a vibration node of the train of ultrasonic waves, so as to bring no discomfort to their propagation.

Considering the drawings, it can be seen that the ultrasonic generator 10, the amplifier 7, the sonotrode 6 form an integral assembly, movable axially under the action of the jack 13. Under the action of this jack, the sonotrode 6 slides through the gland 5 in a sealed manner, and allows the operations of immersion and emersion of the butt 26, in and out of the bath of molten filler metal.

To braze a butt-tube assembly such as 26, we first bring the molten metal bath to a molten state, the upper level of which delimits the brazing zone. The cooler 4 being supplied with pressurized air through the tube 19, the active end 16 of the sonotrode is made to emerge under the action of the jack 13. The assembly 26 is then placed in the cavity 17 where it rests under its own weight.

The sonotrode is then lowered to its submerged position under the action of the jack 20, the temperature is allowed to rise, then the ultrasonic generator is started for a time sufficient to ensure pickling, without harmful corrosion of the parts. to be brazed and their wetting by the filler metal. The sonotrode carrying the brazed stock is made to emerge again, under the action of the jack 13, the stock-tubes assembly is moved away from the sonotrode and allowed to cool until the solder hardens.

By way of practical example, a titanium sonotrode of the commercial grade T40 can be used, with a diameter of about 40 mm, for a length of about 250 mm. An ultrasonic transmitter of the piezoelectric type is used which emits ultrasound at a frequency of approximately 20 kHz at a power of 1100 W, with a nominal amplitude of vibration of approximately 11.2 μm.

For aluminum tubes of approximately 10 mm outside diameter and a 34/66% binary zinc-cadmium filler metal melting between approximately 265 and 305 ° C, the heating time is of the order of 40 seconds, while the ultrasonic emission is approximately 5 seconds.

Of course, the invention is in no way limited to the embodiment described and shown, it is capable of numerous variants accessible to those skilled in the art, depending on the applications envisaged and without departing from the scope of the invention. 'invention.

Thus, depending on the applications envisaged, any suitable conventional sonotrode and ultrasonic emitters with magnetostriction, electrostriction, as well as quartz, of different wavelengths can be used. It is also possible to arrange a device according to the invention for the simultaneous brazing of several assemblies. Any filler metal suitable for particular applications can also be used.

Claims (8)

1. Process for fluxless brazing of metal members (26) in which the members immersed in a molten filling-metal bath (25) are subjected to ultrasonic action before being withdrawn from said bath and being allowed to cool down, characterized in that said members (26) are set in intimate contact with an ultrasonic transducer (6) and said members and ultrasonic transducer are together immersed in the molten metal bath prior to emission of the ultrasounds by said ultrasonic transducer.

2. Process according to claim 1, characterized in that at least that part (16) of the ultrasonic transducer (6) which is destined to be in contact with the filling-metal bath is of unalloyed titanium which is passivated through thermal oxidation previously to the first utilization.

3. Apparatus for the brazing in accordance with claim 1 or 2, comprising a frame (1) supporting a crucible (2) for a molten filling-metal bath, the bottom of said crucible being penetrated in a fluid-tight manner by an ultrasonic transducer (6) displaceable into and out of the bath by means of a jack (13) secured on said frame, an ultrasonic generator (10) associated with said ultrasonic transducer, and controlled heating means associated with said crucible, the active end (16) of said ultrasonic transducer being hollowed out by at least one depression (17), the apparatus being characterized in that said depression (17) is of shape complementary to at least a part of the external surface of the members to be brazed and forms a cavity of dimensions slightly greater than those of the outer envelope of the metal members (26) setting said members in intimate contact with the ultrasonic transducer.

4. Apparatus according to claim 3, characterized in that the passage of the ultrasonic transducer through the bottom of the crucible is located, in the active position of said ultrasonic transducer, at a node of ultrasonic vibrations.

5. Apparatus according to claim 3 or 4, characterized in that the passage of the ultrasonic transducer through the bottom of the crucible is provided with fluid-tight packings of material of high coefficient of ultrasonic absorption.

6. Apparatus for the brazing of the curved portions of U-tubes, according to any one of claims 3 to 5, characterized in that the depression (17) hollowed out in the active end of the ultrasonic transducer has the form of a semi- toroidal U-generated cavity of dimensions slightly greater than those of the outer envelope of said curved portions.

7. Apparatus according to any one of claims 4 to 6, characterized in that at least the active end (16) of the ultrasonic transducer is of passivated unalloyed titanium.

8. Apparatus according to any one of claims 3 to 7, characterized in that the ultrasonic generator (10) is connected to the ultrasonic transducer (6) by an intensifier (7) constituted by a shaped metallic bridging member.

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